Generally, slot dies includes die lips that form an applicator slot. The width of the applicator slot can extend about the width of a moving web or the width of a roller that receives the extruded product, such as a film. As used herein, with respect to slot dies and components of slot dies, “a width” refers to the cross-web (or cross-roller) dimension of a slot die and its components. In this regard, an applicator slot of a slot die extends about the width of the slot die.
Slot dies are commonly used to form extrusions and coatings. As an example, slot dies are used in slot die coatings to apply a liquid material to a moving flexible substrate or “web.” There are many variations in techniques for slot die coatings. As one example, coating materials can be at room temperature or a controlled temperature. When a coating material temperature is elevated to ensure that the coating material is melted or liquefied for processing, this is often referred to as “hot melt” coating. In other examples, a coating material can include solvent diluents. Solvents can be water, organic solvents, or any suitable fluid that dissolves or disperses components of a coating. Solvents are typically removed in subsequent processing such as by drying. A coating can include single or multiple layers, and some slot dies may be used to apply multiple layers simultaneously. A coating can be a continuous coating across the width of the die or instead include form strips, each strip extending across only a portion of the width of the die and being separated from adjacent strips.
Slot dies are also used to form extrusions, including thin-film extrusions or other extrusions. In some examples, extrusions can be extrusion coatings and applied to a web substrate, a process which may be referred to as extrusion coating. In other examples, the extruded material forms a film or web directly. An extruded film might be subsequently processed by length orienting or tentering operations. As with coating, the extrudate might comprise a single layer or multiple layers.
The thickness of an extruded product, such as a film or coating, is dependent upon, among other factors, the flow rate of the extrudate through the slot die. In one example, a slot die can include an adjustable choker bar within the flow path that can be used to locally adjust the flow rate of the extrudate through the slot die to provide a desired thickness profile. A slot die can also include a flexible die lip that can be used to locally adjust the thickness of the applicator slot itself to control the flow rate of the extrudate from the applicator slot to provide a desired thickness profile.
A slot die may include a plurality of actuators spaced about the width of the applicator slot in order to provide a desired thickness profile for an extruded product. For example, each actuator can be configured to provide a local positional adjustment of a choker bar or flexible die lip.
After starting an extrusion process using a slot die, the cross-web profile of an extrudate can be measured. Each actuator may then need to be individually adjusted to provide a desired thickness profile, such as a consistent thickness, for the extruded product across the width of the applicator slot.